Production and investigation of nuclear track membranes at JINR

Nucl. Tracks Radiat. Meas., Vol. 19, Nos 1-4, pp. 919-924, 1991 Int, J. Radiat. AppL lnstrum., Part D Printed in Great Britain PRODUCTION AND INVESTIGATION OF NUCLEAR TRACK M E M B R ~ 0735-245X/91 $3.00 + .00 Pergamon Press plc AT JINR V.I.KUZNETSOV, A.YU.DIDYK, P.YU.APEL Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research Dubna, U.S.S.R. ABSTRACT In this review the status of the technical and scientific problems associated with the nuclear track membranes is briefly given. The main results obtained in the recent years are presented. KEYWORDS Nuclear track filters, membranes, polymers, heavy ions. INTRODUCTION The formation of small pores of controlled geometry by chemical etching of nuclear tracks in dielectrics has been first described by Price and Walker (1962). Immediately after the discovery of Price and Walker, the develo~nent of fission tracks by etching was used in nuclear physics experiments perfcrmedat the Laboratory of Nuclear Reactions of JINR (Perelygin et al., 1963). In the early seventies work on nuclear track membrane technology ("nuclear filters") by using heavy-ion accelerators was started at the LNR. After a period of research and development, a regular production of microporous membranes of polyethylene terephthalate (PETP) was arranged. Nuclear track membranes (NTM) have some significant advantages over conventional membranes. However, certain characteristics of the NTM (such as chemical resistivity, permeability, etc.) need to be improved. PRODUCTION OF PETP NUCLEAR TRACK MEMBRANES For many years the cyclotron U-300 was being used for irradiation of PETP films 10 pm thick and 320 mm wide. Usually, the film band was irradiated by Xe ions with an energy of ca. I MeV/nucleon. Sometimes other ions (Kr, Co, Ar) were used.In 1989 the U-300 was stopped and now the cyclotrons IC-100 and U-400 are operated. A description of an irradiation facility using a scanning ion beam was presented earlier (Flerov, 1984; Flerov et al., 1988). The accelerated ion beam is passed horizontally over the film area by a pair of electrostatic deflection plates. Simultaneously, a foil band is pulled continuously through the beam with a speed ranging from several cm to several meters per second. The speed of the film movement is adjusted to the beam intensity and to the pore diameter and porosity which are required. After ion bombardment, the PETP film is exposed to U.V. light with a wave length ranging from 300 nm to 400 nm (Tretyakova et al., 1980). The film is kept in the air at room temperature until etching. The etching is performed by using NaOH solution of various concentrations. The temperature and Na0H content are chosen depending on the pore size required. The membranes produced are tested by means of the followed set of metods: - the bubble point technique, - by measuring the flow rate of nitrogen (or air) and water, - by measuringthe burst strength (the pressure which breaks I cm z unsupported membrane), - scanning electron microscopy. The conductometric technique and some other methods are use: in the research and development phase. Membranes with a pore diameter between 0.015 ~m to ca. 10 ~m can be produced with porosities from few per cent to ca. 15%. 919 920 V . I . KUZNETSOV et al. Fig. I. The equipment used for the irradiation(abov@and etching~elow)of polymer films. TRACK STRUCTURE AND HEAVY-ION INDUCED RADIATION EFFECTS IN POLYMERS A study of the latent track structure is of key importance in the production of N~i ~rith small pores. Detailed investigations of the track structure have been carried out using electrical conductance monitoring during the etching process, polymer solutions viscosity measurements, sol-gel analysis of polymers irradiated, U.V. and I.R. spectroscopy (Apel et al., 1984; Apel and Pretzsch, 1986; Apel at al., 1987; Apel et al., 1988; Apel and Kravets, 1989). The transversal size of the highly destroyed track core which can undergo selective etching was determined. It was found that around the track core a halo exists where the cross linking of macromolecules may predominate over the chain scission. In Fig. 2 a diagranm is presented which shows the spatial distribution of radiation chemical effects in the track in PETP compared with the track structure given by Chatterjee and Magee (1980). Many of the cross linking events are located at large distances from the ion path. This leads to the modifying properties of a polymer matrix between individual tracks and to the formation o ~ a cross linked network in polymers under ion bombardment at relatively low doses ( 4 10T M Xe ions/cm 2) From Fig. 2 one can see that the region of cross linking and that of the electronic collision cascade coincide. Nevertheless, from experimental data it seems more plausible that the chemical changes in the halo are caused mainly by the effect of active radicals (such as hydrogen atoms) diffusing from the track ccr'e (Apel, 1989). NT MEMBRANE PRODUCTION IN JINR 921 Q b Fig. 2. x~ of a lancet nuclear track in a polymer: a) Schematic model of a track (Chatter Jee and Magee, 1980). The track core (innermost black circle) and the "penumbra" x x y~ x X / X X X )~ X X ~ X x ! 1Onto I are shown. The lines in the "penumbra" indicate individual electron tracks. b) The spatial distribution of chain scission (cirles) and cross link (crosses) events in a Xe ion track in PETP. The approximate numbers of the events per Into of the ion path were calculated from the data of end-groups measurements and sol-gel analysis. STABILITY OF THE PORE SIZE, THE SURFACE AND CAPILLARY PHENOMENA The problem of stability of porous structure in nuclear track membranes with small pores was studied. It was revealed that the process of drying PETP NTM is accompanied by disappearance of pores with a diameter less than 15 nm. The phenomenon is conditioned by the forces of capillary contraction (Apel et al., 1985). Furthermore, it was observed that the critical size of pores which can disappear depends on the properties of the initial m~terlal, on the conditions of the etching process and the storage of a membrane (Andrassy and Mamonova, 1989). The alternation of porous structure of the membranes has also been investigated by heating dry membranes in a temperature range of 333 K to 473 K. It was found that the pore radius can both increase and decrease after the treatment at increased temperature. The analytical equation based on the Alfrey mechanical approach to the relaxative deformation of polymers describes satisfactorily the experimental data (Ovchlnnikov, 1989). The behaviour of the porous structure by applying external tensile stress to the NTM has been studied. The loading of the membrane by a pressure difference exceeding some threshold value leads to a considerable pore growth reaching hundreds per cent. The behaviour of the membrane in the lo~ding a~d. offloading cycles was explained in terms of the viscoelastic deformation consisting of the elastic, elastic time-la@ and plastic components (0vchinnikov et al., 1989). One has to take into account the phenomena observed when the membranes with small pores are employed in practice. Another important property of NTM made of PETP is the surface electrical charge. The electrical charge on the pore wall surface of the membranes in electrolyte solutions is formed mainly due to the dissociation of the surface carboxylic groups. The concentration of fixed charges on the pore walls is close to I elementary charge per nanometer squared (Apel and Pretzsch, 1986; Andrassy and Mamonova, 1988). The smaller the pores, the more marked is the influence of the surface charge on the properties of the membrane. MEMBRANES MODIFIED BY RADIATION-INDUCED GRAFTING Such membranes are prepared by the radiation-lnduced grafting of monomer compounds (styrene, vinylpirrolydone). Three methods of grafting are used: simultaneous gamm~ irradiation and grafting, preirradiation in a vacuum, and prelrradiation in the air (Zhitariuk et al., 1988; Zhitariuk and Kuznetsov, 1989). The monomers have different polymerization rates onto nuclear membranes with different pore diameters but approximately constant porosity (4-10%). The grafted chains localize mostly in polymer m~trix between pores. After the grafting process the membrane thickness increases whereas the pore diameter somewhat decreases. Inhibition of monomer grafting by oxygen indicates a radical mechanism of the process. Locn..li~stion of the grafted polymer in the volume and on the pore walls of a membrane influences its as water permeability, the latter depends o~, th~ r,~ture of the grafted polymer (Zhltariuk et al., 1989; Zhitariuk and Stackc, 1989). Using the radiation-induced grafting, one can change both 922 V.I. KUZNETSOV et al. the bulk and surface properties of the nuclear track membranes. NUCLEAR TRACK MEMBRANES MADE OF CHEMICALLY RESISTANT POLYMERS The resistance of the PETP membranes against aggressive chemicals is limited. Therefore, the polypropylene (PP) and polyvinylideneflucride (PVDF) films were tested for the production of nuclear track membranes. The biaxially oriented isotactic PP films 10 ~m thick were irradiated with accelerated heavy ions and etched in a solution containing sulphuric acid and Cr (VI). Because of the anisotropy of the PP films dne cross section of pore channel is always elliptic in shape (Flerov et al., 1989). The pores consist of two cones (the shape of a "sand-glass"). The heavier the bombarding ion the smaller is the cone angle and the more cylindrical are the pore channels. In Fig. 3 the surface of a polypropylene membrane with large holes is shown. The radiolysis and etching behaviour of PVDF and copolymer of PVDF with tetrafluoroethylene were also investigated (Tretyakova et al., 1986; Tretyakova and Shirkova, 1988). It was found that the 13% Na0H + KMn04 (saturated) solution provides the highest etching rate. The results obtained indicated that these polymers may be used as nuclear track membrane matrices. Fig. 3. SEM-photograph of the nuclear track membrane made of polypropylene "torayfan" (Toray, Japan). SOME APPLICATIONS OF THE NTM's Now the nuclear track membranes are widely used in the U.S.S.R. The most typical industrial application of the NTM is the microfiltration of ultra-pure delonized water used in microelectronics manufacturing (Flerov, 1984). The NTM are effectively used in analytical application, for instance, in medicine (Flerov et al., 1987). The possibility of organizing superfine air cleaning in the SMIF system of contemporary microelectronics technology is investigated (Vorobiev et al., 1989). With the collaboration of specialized scientific and industrial organizations, we develop many other applications of NTM (Belushkina et al., 1989; Mitrifanov and Apel, 1989; Ovchinnikov et al., 1990). ACKNOWLEDGEM~ The authors express deer appreciation to Academician G.N. Flerov and Professor Yu.Ts. 0ganessian for their support and attention to the present work. 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